Fissionable fuel grade uranium oxides for service in power generating nuclear reactors are commonly produced from uranium hexafluoride. A basic chemical procedure practiced in the industry for commercially carrying out the chemical conversion of uranium hexafluoride to uranium oxides for reactor fuel is commonly referred to in this art as a "wet" process. The process is "wet" in the sense that the conversion reactions are effected by or carried out within an aqueous medium or liquid phase with the reactants in solution and/or as a solid suspension therein. Typically, this so-called wet process comprises hydrolyzing uranium hexafluoride (UF.sub.6) in liquid water to form the hydrolysis product uranyl fluoride (UO.sub.2 F.sub.2), adding ammonium hydroxide to the uranyl fluoride to precipitate the uranyl fluoride as solid ammonium diuranate ((NH.sub.4).sub.2 U.sub.2 O.sub.7), then dewatering the solids and calcining in a reducing atmosphere to produce an oxide of uranium (e.g., UO.sub.2). This version of the wet process is frequently referred to as the "ADU" procedure since it normally entails the formation of ammonium diuranate.
The uranium oxides commercially produced by such conventional methods comprise a fine relatively porous powder which is not suitable as such for use as fuel in a nuclear reactor. Typically, it is not a free-flowing, relatively uniform-sized powder, but rather clumps and agglomerates of particles of varying sizes, making it unsuitable to uniformly pack into units of an apt and consistent density. These uranium oxide powders often have very high particle surface areas.
Thus, the raw uranium oxide product derived from the chemical conversion is normally processed through conventional powder refining procedures, such as milling and particle classification, to provide an appropriate sizing of the powders. Such processing frequently includes blending of uranium oxide powders of different particle sizes or ranges and from different sources. Commonly the powdered uranium oxides are handled and conveyed through such processing operations by pneumatic means. Thus, the uranium oxides can be subjected to extensive exposure to air, and in turn, oxygen.
Aptly processed uranium oxide powders are press molded into "green" or unfired pellets which are subsequently sintered to fuse the discrete powder particles thereof into an integrated body having a unit density of 95 to 97% of theoretical ("TD") for the oxide of uranium, and suitable for utilization in the fuel system of a nuclear reactor.
Uranium dioxide is an exception to the law of definite proportions since "UO.sub.2 " actually denotes a single, stable phase that may vary in composition from UO.sub.1.7 to UO.sub.2.25. The thermal conductivity of uranium oxide decreases with increasing oxygen-to-uranium (O/U) ratios. Thus, uranium dioxide having as low an O/U ratio as practical is preferred for use as fuel in nuclear reactors to enable the most efficient passage of heat generated within fissioning fuel material outward to an external heat transfer medium. However, since uranium dioxide powder oxidizes readily in air and absorbs moisture, the O/U ratio of the powder tends to increase significantly to an excess of that acceptable for use as nuclear fuel for effective operation of a nuclear reactor.
During the foregoing chemical conversion process, UO.sub.2 scrap materials, such as sintered pellets, green pellets and calciner powder, are produced. These materials are conventionally recycled. Usually, scrap UO.sub.2 materials from the production facility are oxidized in a high-temperature furnace to produce U.sub.3 O.sub.8 and then the U.sub.3 O.sub.8 is reacted with nitric acid to produce uranyl nitrate solutions. Uranium can be precipitated from these solutions with ammonium hydroxide to produce ADU. The ADU precipitate may or may not be dried, before processing through the calciner in a hydrogen reducing environment to produce UO.sub.2 powder.
Usually this UO.sub.2 powder has a low sinter density, less than 10.60 gm/cm.sup.3 or 96.6% TD, as shown in FIG. 7 of an article entitled "The Influence of Precipitation Conditions on the Properties of Ammonium Diuranate and Uranium Dioxide Powders" by J. Janov et al., Journal of Nuclear Materials, Vol. 44, pp. 161-174 (1972). Other sintered pellet characteristics include high open porosity, nonuniform microstructure, with poor production yields, that is, radial cracks, end flakes, etc. Janov et al. attributed this to the large agglomerates formed during the ADU precipitation step.
In particular, Janov et al. found that the pH at which precipitation occurred was the most important parameter in determining the size of ADU agglomerates and the settling rate and filterability of the slurry. In two-stage precipitation, the ADU properties were determined by the proportion of uranium precipitated at different pH values.
Specifically, Janov et al. reported that the physical nature of ADU, as well as its chemical composition, changes with pH of precipitation. The sizes of ADU crystallites and agglomerates both decrease with increasing pH of precipitation, resulting in a decrease in the filterability and settling rate of ADU slurries. The most filterable ADU was produced at pH 3.5, where a plateau exists in the uranyl nitrate-ammonium hydroxide titration curve. The ADU is partly soluble in this region and large crystallites and agglomerates are formed. However, all the uranium is not recovered from solution as ADU until pH 6-7 is reached.
Janov et al. found that the size of UO.sub.2 agglomerates was determined by the manner in which the parent ADU had been precipitated. Reduction at about 600.degree. C. chemically converted the ADU to UO.sub.2 and caused changes in crystallite size, but the agglomerates remained essentially intact. The size of the UO.sub.2 agglomerates was governed primarily by precipitation conditions. In turn, the size of agglomerates in the UO.sub.2 powder was found to be a more important parameter than surface area in determining powder sinterability. There was no general correlation between the UO.sub.2 surface area and the sintered density of the UO.sub.2 pellets.
Janov et al. further concluded that the greater the amount of uranium precipitated at pH 3.5, the greater was the proportion of large agglomerates in the ADU and the ensuing UO.sub.2 powder. As the percentage uranium precipitated at pH 3.5 increased above 75%, the sintered density achieved with UO.sub.2 powders derived from the ADU decreased rapidly. The large agglomerates present in poorly sinterable UO.sub.2 powder affected the microstructure of the sintered pellets. Pellets fabricated from UO.sub.2 containing only small agglomerates have much smaller grains and generally a denser pellet with uniform microstructure is obtained. Powders containing large agglomerates gave sintered pellets with low densities and non-uniform microstructures.